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Originally Posted by Augi
@ Martian:
OK. So scientists haven't proven without a doubt that it is a black hole? So what do they call objects that cannot be seen and have stars whizzing around them?
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Well, that's the big question, isn't it?
First off, it's really not as simple as 'objects that can't be seen.' There are all kinds of stellar objects that we either haven't found or can't detect. It all has to do with luminosity.
The basic premise is that bright objects make less bright objects near them harder to detect. Like most of the basic premises in science, this is intuitive; if you place a candle in front of a search light, you're probably not going to be able to see the candle.
On top of that, it's incorrect in a technical sense to say that one object orbits another, as no one object is fixed in space. Rather, it's more accurate to say that two objects interact. In the case of a very small object interacting with a very large object (the Earth with the Sun, for example) the practical result is that the large object stays in place, since the gravitational pull of the smaller object isn't very often sufficient to cause a significant movement in the larger one. In the case of two objects that have a lot of mass, on the other hand (say, a black hole and a star), things get a bit more complicated. Making it worse is that none of this stuff exists in a vacuum and nearby massive objects will also exert gravitational pull, resulting in much more complex equations.
I'm really not going to get into it all here; it's fascinating stuff, but there's a whole big pile of background theory that has to be explained before we can even begin to explain alternate possibilities of what's going on here. Suffice to say, however, that even though we're nearly entirely sure that black holes exist and are affecting other objects in space, we can't be entirely sure, and that as long as we have alternate explanations and no direct evidence regarding the existence of black holes they do remain hypothetical. If you're interested in learning more about it all, Hawking's work is a good place to start, but to understand that you're probably going to have to also brush up on Kepler and special relativity and... well, you get the idea. We do have evidence that points to their existence and even gives us some idea as to what happens around them, and it's not all gravitational (or rather, it is, but it's more complex than just 'it pulls everything towards it'). Dr. Hawking in the first edition of A Brief History of Time (the very first book I ever read on the subject, as is the case for a great many people) stated that we were "95% certain" in 1988; I'd go as far as to say we are 98 or even 99% certain now. It's just that little 1% that gets in the way.
Understand that there are few if any actual physicists out there who are going to argue against the existence of black holes. Since Hawking conceded his bet with Thorne, nobody really feels confident in saying that these things don't exist. On the other hand, until we have incontrovertible proof, we cannot scientifically claim that black holes do exist; hence my statement above that they are still technically hypothetical.